Thermal-flattening of metallic strip



pril 21, 1964 L. cooK THERMAL-FLATTENING OF METALLIC STRIP INVENTOR. Lowe-L L. (ba/f, 13% 4%9 TTORNYS.

United States Patent 3,136,088 HERMALFLA'ITENING 0F METALLIC STRIP Lowell I.. Cook, Butler, Ia., assigner to Armco Steel Corporation, Middletown, (Ehio, a corporation of Ohio Fiied Dec. 3l, 1958, Ser. No. 734,214 2 Claims. (Ci. 14S-12) This invention relates to the thermal-attening of metallic strip and more particularly to the tlattening of continuous steel strip which is the product of either tandem or reversing cold mills or composed of butt welded sheets. The invention will nd particular utility in the flattening of steel that is to be used for transformer or generator laminations wherein liatness is of the utmost importance. It will be understood, however, that its utility is not so limited and the invention may be utilized in other applications wherein improved ilatness characteristics are desired.

A conventional method of thermal-nattening is described in U.S. Patent No. 2,412,041 of which I am a co-inventor. As taught therein, the flatness characteristics of hot rolled sheets may be usefully improved by applying controlled tension to a strip as it is drawn through a furnace on carrier rolls. However, with this procedure the degree of improvement will vary substantially with the shape of the strip being acted upon. There are a number of defects, such as deformed edges, i.e. wavy, ared and/or buckled edges, and rows of pockety buckles within the body of the strip (as opposed to its edges), which cannot be removed by this conventional method. Rather, I have observed, for example, that the application of tension to a strip having a series of pockety buckles results in what is known in the industry as canoeing or the formation of longitudinal corrugations or troughs. I have further observed that such longitudinal corrugations can also be produced by excessive tension regardless of the shape of the entering strip. It is to the elimination of the aforementioned types of defects that the instant invention is directed.

It is a principal object of my invention to provide a procedure whereby the fiatness characteristics of metallic strip may be materially improved, with particular emphasis on the elimination of longitudinal corrugations which have characterized conventional methods of attening.

A further object of my invention is the provision of a thermal-attening treatment by means of which metallic strip having badly deformed edges may be treated to produce an extremely at product; and by the term extremely flat, I means that the deviation in a 3l" wide strip will be of the order of .05 inch or less.

Still a further object of the invention is the provision of a flattening treatment particularly applicable to cold rolled steel including the electrical steels irrespective of their silicon content or their content of other alloying ingredients.

Still a further object of rny invention is the provision of a iiatness treatment which is not only relatively inexpensive but which may be practiced upon magnetic materials without imparing the magnetic characteristics developed in such materials by other processing techniques.

The foregoing objects, together with others which will be pointed out hereinafter or will be apparent to the skilled worker in the art upon reading these specifications, I accomplish by that treatment and procedure of which I shall describe an exemplary embodiment.

ICC

Reference is now made to accompanying drawing wherein:

FIGURE 1 is a schematic longitudinal sectional view of a flattening furnace in accordance with the invention.

Essentially, my invention contemplates a positive flattening of the strip by means of a furnace in which the strip is threaded over and under alternate spaced apart attening rolls, the strip being drawn through the furnace under tension with the rolls in positive contact with the strip across its full width as it is drawn through the furnace. The amount of work done on the strip is the minimum that is required to achieve the desired atness. To this end such factors as roll spacing, the number of attening rolls, roll diameter, top roll penetration, strip tension, strip speed, and furnace temperatures are controlled in such a way that their combined effect will produce the desired degree of iiatness without adversely affecting other properties of the steel. This is particularly important in the case of grain oriented silicon steel wherein excessive working of the strip would be detrimental to its magnetic properties. With my invention, the length of the attening furnace can be materially reduced, thereby reducing the cost of the flattening line.

My invention is based upon the observation that the buckling or canoeing of a metallic strip bears a direct relationship to its slenderness ratio; and that by maintaining the slenderness ratio as low as possible, the strip will be essentially freed from buckles and other irregularities tending to adversely aect its flatness. The slenderness ratio of a strip indicates the magnitude of the contracting forces acting to reduce the width of a given length of strip when such given length is stretched, i.e. placed under longitudinal tension. For example, if a strip of steel is passed between spaced apart pairs of pinch rolls, with the second or exit set of rolls operating at somewhat greater lineal speed than the iirst or entrance set of rolls, the strip will be stretched or tensioned between the two sets of rolls. The distance between the entering and exit sets of rolls I Will call the gauge length of the strip. The slenderness ratio of the strip is deiined as the ratio of gauge length to the square root of the crosssectional area of the strip. For strip of constant width and thickness, the slenderness ratio thus varies directly with the span over which it is stretched or, in this case, with the distance between the two sets of rolls.

In a conventional attening by tension operation, such as in accordance with the aforementioned patent the gauge length of the strip may be as much as several hundred feet and the slenderness ratio of the strip is quite high. I have found that by reducing the gauge length of the strip, and hence its slenderness ratio, the strip is placed under lateral restraint which effectively reduces the transverse stresses which cause buckling.

In accordance with the instant invention, the rolls within the furnace through which the strip is threaded serve to eiectively shorten the gauge length by dividing the distance between the entrance and exit pinch rolls into a series of relatively short gauge lengths. This materially reduces the slenderness ratio of the strip and the lateral forces tending to buckle it are practically eliminated. At the same time a positive flattening action is exerted by the attening rolls which enable me to materially reduce the length of the furnace.

Referring now to the drawing wherein I have schematically illustrated a mechanism for carrying out the flattening operation, the metallic strip to be attened is fed from a reel or roll stand 1 which is of conventional construction. It will be understood that the strip will be delivered to the reel in coiled condition either from storage or from some preceding operation in its manufacture. Alternatively, it is possible to feed the strip directly from some other operation, as for example a pickling treatment or coating operation, the strip being fed continuously from such other operation directly between a set of entrance pinch rolls 2 which will be driven and will serve to feed the strip Vfrom the feed reel 1 into the furnace. In the event the strip is to be fed directly from a preceding operation, the, set of pinch rolls 2 will be synchronized with the exit speed of the strip from such preceding operation.

The furnace is indicated at 3 and will comprise a heating and soaking section 4 and a coolingsection 5. At the exit end of the furnace, the strip, which is indicated by the. reference numeral 6, passes between the nip of a second set of driven pinch rolls 7 which are driven at a speed somewhat greater than the speed of pinch rolls 2 and hence act to draw the strip through the furnace under tension. Within the furnace, the strip is conveyed on spaced apart carrier rolls S excepting for that portion of the furnace wherein the strip passes alternately between the staggered series of flattening rolls 9a and 9b. Preferably, the bottom flattening rolls 9b, i.e. the rolls which lie beneath the strip and contact its undersurface, will be in alignment with the carrier rolls 8, whereas the top attening rolls 9a will be adjustably mounted so that they may be retracted in order to facilitate threading of the strip and also toV permit their adjustment for proper strip penetration. Theflattening rolls can be either driven or idled, although I prefer the latter. However, it is desirable for all rolls within the furnace to be provided with means for turning them while heating up the furnace after a shut-down in order to prevent roll warpage.

InV an exemplary installation now in operation, the diameter of the attening rolls is three inches and they are spaced apart a distance of 1 l0". The strip passes between two top rolls and two bottom rolls, and the top roll penetration or degree of wrap-around is adjusted so that the top and bottom rolls are essentially in line, i.e. the top rolls penetrate the pass line of the bottom rolls by a distance substantially equal to the diameter of the rolls.

In other installations now in use the attening rolls have a diameter of 4% inches, they are spaced apart by a distance of from 4 to 6 feet andthe stripY is passed between asV many as four sets of flattening rolls. As before, the top rolls are adjustable and they may be set to penetrate the pass line of the lower rolls by a distance up t the diameter of the roll. It will be understood, of course, that roll diameter, roll spacing, the number of rolls, and their penetration are interrelated and may be varied depending upon the. gauge and type of strip being handled. An exact rule for the correlation of the various factors involved cannot be given because of theinterplay of many variables, but with any given material it will be within the skillof the worker in the art to establish the necessary conditions to produce the desired atness. It is also readily possible to vary the spacing and penetrationV in a single installation if so desired, although I prefer tokeep them uniform in, each installation. Unlike commercial leveling processes using closely-spaced interdigitated rolls which flatten by flexing the strip, my flattening is accomplished by applying tension while the stripy is restrained laterally by the action of a series of relatively widely spaced rolls making full width contactV with the strip. It is literally impossible to describe the combination of stresses which produce the flattening, because the elevated temperature properties of metals are not well knownv at the present time. Undoubtedly, the phenomenon of creep is involved, as well as the structural instability of metals subjected to high temperatures.

The strip tension may be varied depending upon the type of steel strip being handled, its gauge, and the fur# nace temperature; and I have found that best results for silicon steel, for example, are obtained when the strip tension is from 1100 to 2200 p.s.i. with optimum results at about 1500 p.s.i. and the furnace temperature is about 1500o F.

The temperature of the heat treatment to produce latness is not critical per se and. in most cases will be governed by a consideration of the results to be produced. Essentially, the object is to heat the strip to a temperature at which it becomes soft enough for the relief of strains while at the same time not hurting the other properties of the strip. When Working with magnetic steel the preferred temperature is about 1475 E1-25 F., and care should be taken not to exceed l500 C., since at higher temperatures the magnetic properties of the steel might be damaged. Depending upon the character of the strip, I have found it practical to work with temperatures down to about 900 F. for steel and even lower for other metals, it only being necessary to exceed the softening temperature of the metal being treated. Similarly, where cornmon grades of steel strip are being flattened much higher temperatures may be employed, and if flatness is the only consideration the maximum permissible temperature is essentially determined by mechanical factors.

The length of time it takes to bring the material up to temperature is of course aifected by its gauge. The heat-` ing takes place quite rapidlyV and, once the strip has been brought to temperature, no appreciable holding time is required. I have found it desirable not to cool the strip too abruptly and prefer cooling at a rate slower than a free cooling in air at room temperature. To this end I prefer to employ the cooling hood 5 wherein the temperature drop may be made somewhat more gradual. The atmosphere within the furnace does not affect the flatness of the strip, and hence forms no limitation on the process. If desired, the atmosphere may be chosen in accordance with its effect upon other qualities of the metal, such as brightness, decarburization and mechanical properties. It is preferred that a neutral or reducing atmosphere be employed to substantially prevent scaling of the strip. While the primary purpose of the procedure is to Vproduce llatness in the strip, and to this end the amount of work done on the strip should be the minimum that is required to achieve. the desired flatness, it is nevertheless possible under certain circumstances to concurrently utilize the heat treatment for other purposes. For example, in producing magnetic steels wherein an open anneal may be. desirable for other reasons as one of the steps or as the linal step of a particular routing, furnace temperatures appropriate to said step may be employed concurrently with the use of the proper flattening roll penetration and tension. Such treatments not only produce the desired atness but in addition constitute an integral step in the production of the desired properties in the strip.

Modifications may be made in the invention without de-` parting from the spirit of it. Having, however, described the invention incertain exemplary embodiments, what I claim as new and desire to secure by Letters Patent is:

1. A process for increasing the flatness of steel strip which, in otherwise finished form, is characterized by a lack of flatness, which comprisesA bringing the material to atemperaturel of at least 900 F. at which it is soft enough for the relief of strains, alternately passing theV strip over and undei a plurality of spaced apart flattening rolls some of which at least are arranged to penetrate the` pass line of the strip by a distance no greater than the diameter of said rolls, while at the same time subjecting the material to longitudinal tension, said flattening rolls being spaced from each other by a substantial distance on the order of from. about seven tol fifteen times the diameter of` said rolls, and thereafter cooling the said strip to normal temperature at a rate slower than open cooling in air at room temperature.

2. A process for increasing the flatness of steel strip which, in otherwise nished form, is characterized by a lack. of atness, which comprises bringing the material to a temperature of atleast 900 F. at which it is soft enough for the relief of strains, alternately passing the strip over and under a plurality of spaced apart flattening rolls some of which at least are arranged to penetrate the pass line of the strip by a distance which is no greater than the diameter of said rolls, while at the same time subjecting the material to longitudinal tension of from 1100 to 2200 p.s.i., said attening rolls being spaced from each other by a substantial distance on the order of from about seven to fteen times the diameter of said rolls, and thereafter cooling the said strip to normal temperature at a rate slower than open cooling in air at room temperature.

References Cited in the le of this patent UNITED STATES PATENTS 1,914,439 Langford June 20, 1933 2,333,282 Wilson Nov. 2, 1943 2,432,828 Stone Dec. 16, 1947 2,699,196 Cozzo Jan. 11, 1955 

1. A PROCESS FOR INCREASING THE FLATNESS OF STEEL STRIP WHICH, IN OTHERWISE FINISHED FORM, IS CHARACTERIZED BY A LACK OF FLATNESS, WHICH COMPRISES BRINGING THE MATERIAL TO A TEMPERATURE OF AT LEAST 900*F. AT WHICH IT IS SOFT ENOUGH FOR THE RELIEF OF STRAINS, ALTERNATELY PASSING THE STRIP OVER AND UNDER A PLURALITY OF SPACED APART FLATTENING ROLLS SOME OF WHICH AT LEAST ARE ARRANGED TO PENETRATE THE PASS LINE OF THE STRIP BY A DISTANCE NO GREATER THAN THE DIAMETER OF SAID ROLLS, WHILE AT THE SAME TIME SUBJECTING THE 